1. Field of the Invention
This invention relates to a compression molding method, and more specifically to a method of producing a molded article of a synthetic resin by compression molding a synthetic resin in the heat-melted state. The invention also pertains to an apparatus for feeding a synthetic resin which is used conveniently in the compression molding method, and more specifically, to an apparatus for feeding a synthetic resin comprising an extruder having an extrusion plate with an extrusion opening formed therein and a cutting tool for cutting a molten resin extruded from the extrusion opening.
2. Description of the Prior Art
Plastic containers for holding foods and drinks and plastic closures therefor have been in widespread used. Usually, plastic articles such as containers and container closures are produced by injection molding. The injection molding technique, however, has the following problems or defects as is well known in the art. Firstly, the injection molding has only a limited molding efficiency, and cannot achieve sufficient productivity. Secondly, orientation occurs in the synthetic resin material owing to its flowing during injection. As a result, the molded article does not have sufficient strength or toughness and may undergo breakage in the event of falling. This problem might be solved by increasing the wall thickness of the molded article, but this results in an increase in the amount of the synthetic resin material and therefore in an increase in the cost of production.
On the other hand, it has been proposed, and commercially accepted, to produce container closures of a relatively small size (for example, with an outside diameter of about 30 mm) by compression molding instead of injection molding and therefore to increase productivity. It has been considered to be extremely difficult, if not impossible, to produce molded articles of a relatively large size, for example, container closures with an outside diameter of at least 70 mm, because the synthetic resin material cannot be caused to flow in a required manner by compression, and even when the flowing of the synthetic resin can be effected in a required manner, defects such as creases occur in the peripheral edge portion of the molded article, particularly the outermost peripheral surface. Particularly, in containers having a considerably high side wall, it is necessary to make the thickness of the side wall sufficiently thin (for example, 0.5 mm or below) from the standpoint of cost. If an attempt is made to produce a container having such a side wall by compression molding, the synthetic resin material does not sufficiently flow into a mold space defining the side wall, and the side wall of the required height cannot be formed. The flowability of the synthetic resin material may be increased by selecting a synthetic resin material having a high melt flow index and increasing the compression pressure. However, this does not always lead to fully satisfactory flowability. In addition, higher melt flow indices of the synthetic resin material used generally result in lower strength or toughness. Increasing of the compression pressure, on the other hand, requires a molding apparatus of a larger size, and therefore, the equipment cost increases.
In compression molding, a predetermined amount of a synthetic resin in the heat-melted state is fed into an open mold. Then, the mold is closed under a predetermined pressure to mold the synthetic resin into an article. For feeding the predetermined amount of the synthetic resin into the open mold, there is generally used an apparatus of the type comprising an extruder having an extrusion plate with an extrusion opening formed therein and a cutting tool for cutting the molten synthetic resin extruded from the extrusion opening. The cutting tool is usually adapted to rotate or reciprocally pivot across the extrusion opening. This conventional synthetic resin feeding apparatus has the following problems to be solved.
The synthetic resin extruded from the extrusion opening has a relatively low melt flow index, a relatively low temperature, and a relatively high viscosity. When the amount of the synthetic resin extruded from the extrusion opening for cutting by the cutting tool is relatively small, the extruded synthetic resin is cut properly by the cutting tool, and then leaves the extrusion plate and the cutting tool and falls onto a given site. Experiments conducted by the present inventors, however, have shown that when the synthetic resin to be extruded from the extrusion opening has a relatively high melt flow index or a relatively high temperature and a relatively low viscosity, or when the amount of the synthetic resin extruded from the extrusion opening for cutting by the cutting tool is relatively large, the synthetic resin cut by the cutting tool in the conventional resin feeding apparatus tends to remain attached to the cutting tool and/or the extrusion plate and therefore, be unable to fall onto the given site.